The subject matter of this invention relates generally to protective shutters for the live stationary terminals of metal clad or metal enclosed switchgear and relates more specifically to disconnectable grounding means for composite insulating-conducting barriers.
Metal clad and metal enclosed switchgear are well known electrical apparatus. Generally, a metal enclosed swithgear apparatus includes a metal cabinet having high voltage stationary electrical terminals in the rear portion thereof to which may be connected the terminals of draw-out or roll-out circuit breaker apparatus. The circuit breaker apparatus is generally self-contained and portable to the extent that it can be completely removed from the cabinet for repair, replacement or the like. Prior to operation, the front of the cabinet is opened, the circuit breaker is rolled into position for engagement with the complementary high-voltage terminals and then the cabinet door is closed. At this point, an appropriate manual or motor driven "levering in" operation is begun. The latter "levering in" operation constitutes the final stage in the joining of the stationary high-voltage contacts of the stationary switchgear cabinet with the complementary contacts of the portable circuit breaker apparatus. This is done within the confines of an enclosed metal cabinet for safety. Furthermore, the cabinet is maintained at electrical ground potential for providing a suitable envelope of protection around the high voltage apparatus and terminations contained therein. It had been found previously in the prior art that it was advantageous to somehow shield the high-voltage terminals of the switchgear cabinet from inadvertent physical contact with either persons or other objects when the portable circuit breaker apparatus has been removed. To accomplish this, interlocking means on the cabinet which are complementary with portions of the portable circuit breaker are constructed and aligned in such a way that as the circuit breaker apparatus is removed from the switchgear cabinet, a protective shutter device is automatically deployed in front of the high voltage terminals. Such apparatus is shown in prior art U.S. Pat. No. 3,213,222, issued Oct. 19, 1965 to J. D. Finley et al. and entitled, "Switchgear Shutter". The latter patent is assigned to the assignee of the present invention. In the aforementioned prior art patent, the shutter essentially comprises sheet metal material. The sheet metal material when in place in front of the aforementioned high-voltage terminals essentially provides a complete zero ground plane between the circuit breaker portion of the switchgear cabinet and the stationary terminals of the high voltage portion. It is easy to see that this provides significant protection for personnel who may find themselves within the circuit breaker portion of the cabinet. It can also be seen by examining the aforementioned prior art patent that the main stationary terminals are offset from the shutter by an insulating spout or bottle which comprises a hollow cylindrical insulator into which elongated complementary terminals of the circuit breaker protrude for eventual interconnection with the stationary terminals at the rear of the spout or bottle. The combination of the lowered shutter in front of the opening in the spout and the enlongated insulating tubular barrel provide significant electrical protection. As the prior art developed and the relative size of circuit breaker apparatus and switchgear cabinets were reduced for more efficient utilization of material and space, the longitudinal distance between the shutter at one end of the spout or bottle and the high voltage contacts at the other end began to diminish to a point where flashover between the contacts and the shutter was possible or where flashover between a stored shutter and a live circuit breaker contact was possible. To offset this, the shutters of the more advanced state of the prior art were made of insulating material such as a polyester material which was not electrically conducting and which essentially solved the problem of flashover. However, at this point in the advanced state of the prior art, a significant problem arose. Namely, the polyester material is suitable for insulating the front of the spouts or bottles, i.e., the circuit breaker portion of the switchgear cabinet, from the high voltage terminals at the ends of the bottles, however, the capacitive effect associated with the polyester material acts in such a way that an electrostatic charge can build up or accumulate on a portion of the insulating material so that inadvertent contact therewith from the circuit breaker portion of the switchgear cabinet can be dangerous. On the other hand, the metal shutters are grounded and provided a zero ground plane as was described previously, but are susceptible to arc-over or flashing as was described previously. In the previously mentioned copending cross-related application Ser. No. 6,833, the latter-mentioned problem is solved by utilizing a composite electrically conducting and electrically insulating shutter. The shutter is provided for placement over the ends of the bottles or sprouts of the high voltage stationary terminals of the metal clad switchgear to effectively separate the high voltage compartment of the switchgear from the circuit breaker compartment of the switchgear when the circuit breaker apparatus has been removed. The shutter comprises a significant amount of insulating material which prevents flashover and furthermore comprises judiciously disposed electrically conducting material which provides a suitable electrical ground plane at critical portions of the shutter to provide adequate protection in the vicinity of the bottle openings. Furthermore, the electrically conducting material is grounded only when the shutter is in its protective disposition. The grounding interconnection is removed as the shutter is disposed away from the openings of the spouts for storage, when the circuit breaker terminals are in a position of contact with the high voltage terminals. The ungrounding of the electrically conducting material allows for a relatively small distance of travel in the shutter from its protective position to its stored or non-protective position. Unfortunately, in the prior art suitable means had to be found for effectuating the grounding arrangement. When the grounding strap or grounding bar which is interconnected with the metal clad switchgear is permanently disposed too close to the live electricl parts of the switchgear apparatus, a flashover to the ground bar may occur. On the other hand, if an appropriate grounding mechanism is not provided, the mechanical interconnection at the time when grounding is required becomes a problem. It would be desirous therefore, if grounding apparatus could be found which had the advantage of being disposed close enough to the movable shutter to provide effective grounding at appropriate times but which nevertheless did not cause flashover at a time when grounding was undesirable.